IEEE Robotics & Automation Magazine - September 2023 - 62

on a titanium workpiece is reported in Figure 5(a). As highlighted
in the graph, sound under 1,000 Hz is considered
background noise.
The two visible peaks indicate two distinct modes of the
milling sound: the frequency of the first mode represents the
spindle's dominant frequency, and the value of the second
mode is always double the first mode (first harmonic). Since
the amplitude of the machining sound decreases with the
spindle frequency, it can become undetectable under a certain
threshold (identified as 2,000 Hz in the reported example),
which is affected by the environment noise and microphone
position (a 150-mm distance between the microphone and
machining tool in the reported example). Thus, the distance
× 10-5
1
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
Mode 2 of
Machining
Sound
Mode 1 of
Machining Sound
5
4.5
3.5
4
2.5
3
1.5
2
Environment
Noise
103
Frequency (Hz)
(a)
Receive the Newest PSD Plot
Remove Sound Frequencies Lower Than fen
Select the Frequency With Highest Amplitude as fp,i
fp,i - fp,i-1≤ ∆
Yes
fout,i = fout,i
-1
fout,i-1 - fp,i-1≤
fout,i-1 - fp,i-1/2
No
fout,i = fp,i-1/2
(c)
FIGURE 5. The sound acquisition and processing. (a) An example power spectral density plot at a frequency range from 0 to 20 kHz,
acquired for a titanium sample machined with a milling tool. (b) An example PSD plot with different signal characteristics. (c) The
proposed spindle frequency tracking algorithm.
62 IEEE ROBOTICS & AUTOMATION MAGAZINE SEPTEMBER 2023
Yes
fout,i = fp,i-1
No
104
0.5
1
Environment
Noise
Mode 2 of
Machining Sound
between the microphone and sound source should be minimized
when possible to improve the detection rate.
To enable the real-time detection of spindle frequency variation,
the power spectral density is refreshed at 475 Hz. This refresh
rate is determined by the need for acquiring enough samples to
calculate the PSD frequency plot and cannot be increased further
without deteriorating signal quality. The spindle frequency is
identified in every frame of the power spectral density. This task
is fairly easy when the PSD frequency plot is always as clear as
the example in Figure 5(a). However, undesired spindle vibrations
(e.g., repeated collisions between the machining tool and workpiece,
due to compliance in the fixtures), the natural frequencies
of the workpiece, and the noise produced by the exhaust air might
× 10-6
103
104
Frequency (Hz)
(b)
Power Spectrum Density
Power Spectrum Density

IEEE Robotics & Automation Magazine - September 2023

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2023

Contents
IEEE Robotics & Automation Magazine - September 2023 - Cover1
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IEEE Robotics & Automation Magazine - September 2023 - Cover3
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